1. Field of the Invention
The present invention relates to a solar cell for use as a power supply for a variety of electronic equipment or a power source for a power plant. More particularly, the invention relates to a solar cell having an improved electrode structure with excellent resistance to the environment. Also, it relates to a solar cell which has a cheap, flexible, and durable electrode grid.
2. Related Background Art
In recent years, there has been growing interest in the environment and energy sources because of global warming and radioactive pollution due to atomic power plant accidents. In view of these situations, solar cells have been expected to be exploited as a reproducible, inexhaustible source of clean energy. At present, three types of solar cells are well known: single crystal silicon, polycrystalline silicon, and amorphous silicon. The amorphous silicon type solar cell has excellent characteristics such as its thin film construction, which permits easy realization of large area cells, and a large light absorption coefficient, unlike the crystal type solar cells, even though it is inferior in conversion efficiency to the crystal type solar cell. Thus, the amorphous silicon type solar cell is one of the more promising types of solar cells. Since its cost is estimated to be significantly less than the crystal types if production reaches several hundreds of MW, many studies on it have been performed throughout the world. An example of a conventional amorphous silicon type solar cell is shown in FIG. 2. A photoelectric conversion semiconductor layer 103 made of amorphous silicon is formed on an electrically conductive substrate 104, and a transparent electrically conductive layer 102 also useful as an anti-reflection layer is formed thereon. On the transparent electrically conductive layer there is formed a grid electrode 101 as a current collector. If light 106 is incident on the photoelectric conversion layer 103 from the grid electrode 101, as shown in FIG. 2, light energy is converted into electric current within the conversion layer 103, and outputted through the transparent electrically conductive layer 102 via the grid electrode 101 and the electrically conductive substrate 104. The photoelectric conversion layer 103 contains at least one or more pin or pn junctions, with the p side acting as the anode and the n side as the cathode.
When the solar cell is used outdoors, particularly good characteristics with respect to environmental resistance are required. However, studies by the present inventor have revealed that in a conventional grid electrode, a short-circuit may take place between the electrodes of the solar cell due to water permeating voids in the electrode, which is one of the causes of a decrease in the conversion efficiency. For example, a conventional grid electrode is disclosed in Japanese Laid-Open Patent Application Nos. 59-167056, 59-167057, and 59-168669. Specifically, as described in Japanese Laid-Open Patent Application No. 59-167056, the grid electrode is constituted of a conductive paste composed of 80 wt % silver and 20 wt % phenolic resin binder, but it is poor in durability and the void volume (as shown at 105 in FIG. 2) may increase with time. With such an electrode, it is difficult to produce a solar cell having no degradation of conversion efficiency when exposed to the environment. This aspect will be described later in detail.
In general, a solar cell having an output of several W or greater is used outdoors. Therefore, so-called "environment proof" characteristics with respect to the temperature and humidity are required. Particularly in a solar cell having a grid electrode as a collector, a conductive material such as silver or the like contained in the grid electrode may dissolve by the permeation of water (as shown with Ag.sub.2 O in FIG. 3) and by the photoelectromotive force of the material, diffuse through defective portions such as pinholes or exfoliations. This causes a short-circuit between the positive and negative electrodes of the solar cell, thereby greatly decreasing the conversion efficiency. For example, when the conductive base material is silver, the reaction proceeds between the anode and the cathode according to the following formula, thereby giving rise to a short-circuit. EQU Anode Ag.sub.2 O and H.sub.2 O.fwdarw.2Ag.sup.+ +2OH.sup.- (A) EQU Cathode Ag.sup.+ +e.sup.- .fwdarw.Ag (dendritic crystal deposition)(B)
This behavior is shown in FIG. 3. A silver ion 305 arising from a positive-side collector electrode 101 enters a pinhole 306 existing in the photoelectric conversion semiconductor layer 103 due to an electric field produced by the absorption of light, and adheres to the conductive substrate 104 to form a dendritic type crystal 307. If the dendritic crystal grows, the collector electrode 101 of the solar cell is electrically shorted to the conductive substrate 104, so that the output of the solar cell is reduced. Consequently, degradation of the conversion efficiency may occur.